In the course of a semiconductor manufacturing process, a semiconductor wafer may be subjected to a variety of processing steps such as for example heat treatment, oxidation or chemical vapour deposition. In order to execute these processing steps efficiently, a plurality of wafers is commonly loaded onto a so called wafer boat, i.e. a carrier for holding the semiconductor wafers in a spaced arrangement during processing in a furnace or reactor. Wafer boats are usually used vertically, that is, the wafers lie horizontally, one above the other. The probably most common type of wafer boat comprises three or more vertically oriented, parallel rods, which extend between a top member and a bottom member. In the rods, vertically spaced recesses are formed at corresponding heights. Each recess serves as a support arranged for engaging a side portion of a wafer, and recesses at the same height define a holding position for receiving and supporting a wafer in a substantially horizontal orientation. To minimize gravitational stress on a wafer, at least two of the rods are located toward a front side of the wafer boat—where the wafers are inserted into and removed from the holding positions—, and at least one rod is located at a back side of the boat. As the arrangement of the rods relative to the wafers is subject to the requirement that it must be possible to insert and remove wafers into and from the wafer boat, the above-described configuration commonly amounts to a wafer boat in which a little less than half of each inserted wafer is unsupported and freely hanging. Due to its own weight, this unsupported part of the wafer tends to sag.
Wafer handling equipment including an end effector for insertion and removal of semiconductor wafers into and from the wafer boat requires that the space between any two consecutively stacked wafers is accessible and allows for some maneuvering of the end effector. For example, in order to pick up one or more wafers from a wafer boat after processing, the end effector must be able to reach the interstitial space(s) between one or more pairs of wafers in order to engage the wafers at suitable positions, such as their respective centers. Sagging of the wafers causes the accessibility of the space between any two wafers to diminish, and reduces the effective room available for maneuvering. Thusfar this problem has been dealt with by increasing the vertical spacing between the holding positions of a wafer boat, which results in a decreased wafer capacity per boat, and thus a reduced process efficiency. It is an object of the present invention to alleviate or overcome aforesaid problem associated with the sagging of the wafers without said shortcomings.